Author Bio

Martyn Gaudion, CEO of Polar Instruments Ltd., began his career at Tektronix in the early 1980s where he was responsible for test engineering on high-bandwidth portable oscilloscopes. Gaudion joined Polar in 1990 where he was responsible for the design and development of the Toneohm 950, Polar's multilayer PCB short circuit locator. He became marketing manager at Polar during 1997 as the market for controlled impedance test became a major section of the company's product range, was appointed Sales and Marketing Director in January 2001 and was appointed CEO in January 2010. Gaudion also writes occasional articles for a number of PCB industry publications and regularly contributes to IPC High-Speed High-Frequency standards development activities. He may be contacted Polar Instruments Ltd., Garenne Park, Guernsey, UK, GY2 4AF; phone:+44 1481 253081; fax: +44 1481 252476

The Pulse: Tangential Thoughts--Loss Tangent Values

Numbers are fascinating things, and the way they are presented can influence our thinking far more than we would like to admit, with $15.99 seeming like a much better deal than $16.00 (though it depends which side of the transaction you sit on!) When looking for a new job, you may prefer to round your existing salary up to the nearest thousand dollars, not down, when speaking with a potential employer. Likewise, a salary of $60,000 sounds better than one of $0.061 million, even though the latter is a larger number. Our brain has been programmed to suppress the importance of numbers to the right of the decimal point.

Such is the case with the loss tangent of materials. It is a tiny number and so to our minds looks insignificant, but it has a directly proportional effect on the energy loss suffered by a dielectric. I am always curious that engineers seem to obsess over dielectric constant, the ability of a substrate to store energy and its effect on impedance, which is a one over root effect, so this is a second order impact on Z0.

Yet engineers go to great lengths to attempt to find the exact value of Er despite its second-order effect on the circuit characteristic impedance. But the loss tangent? Well, it’s a small number, isn’t it? So, why not round it off to a few decimal places? Logical thought is suspended just because it is a small number, but when you are modeling insertion loss, the loss is directly proportional to the loss tangent.

A Practical Example

Because loss tangent is a small number, it is perhaps easy to forgive people who round it off to fewer decimal places. Our minds are wired to dismiss numbers far to the right of the decimal point. However, this can lead to unintended miscalculations when rounding small value parameters such as loss tangent which has a directly proportional effect on the insertion loss. A rounding to 2 decimal places of a TanD say, from 0.015 to 0.02 (quite legitimate, you may think) would actually lead to the modeling of insertion loss being overpredicted by a massive 33%.

You can see the effect when using a PCB field solver’s frequency-dependent calculation feature to model loss in an offset stripline 1B1A controlled impedance structure.

To read this entire column, which appeared in the October 2017 issue of The PCB Design Magazine, click here.

2017

Numbers are fascinating things, and the way they are presented can influence our thinking far more than we would like to admit, with $15.99 seeming like a much better deal than $16. Likewise, a salary of $60,000 sounds better than one of $0.061 million, even though the latter is a larger number. Our brain has been programmed to suppress the importance of numbers to the right of the decimal point. Such is the case with the loss tangent of materials. It is a tiny number and so to our minds looks insignificant, but it has a directly proportional effect on the energy loss suffered by a dielectric.

2016

Remember that good modeling can’t fix a bad design. The model can tell you where a design is weak, but if you have committed your design to product, the model can only tell you how it behaves. Some less experienced designers seem to think a better model will fix something that doesn’t work; it won’t. It will only reassure you that the design was bad in the first place.

2015

The positives for new fabricators and designers lie in the fact that, even though impedance control may be new to them, there is a wealth of information available. Some of this information is common sense and some is a little counterintuitive. So, this month I’d like to go back to the fundamentals, and even if you are an experienced hand at the subject, it can be worth revisiting the basics from time to time.

In my December 2013 column, I discussed “rooting out the root cause” and how sometimes, the real root cause is hidden when digging for the solution to a problem. In that column, I described how sometimes in an attempt to better correlate measured impedance with modelled impedance, fabricators were tempted to “goal seek” the dielectric constant to reduce the gap between predicted and measured impedance.

Like the whack-a-mole game where the moles keep popping up at random after being knocked back into their holes, the same old questions about technical hurdles surrounding signal integrity continue to surface as technology advances.

2012

One of the more popular editions of The Pulse in 2011 was the article "Transmission Lines - a Voyage From DC." Starting again from DC and working through the frequency bands, Martyn Gaudion looks at what is realistic to achieve and where economic compromises may need to be made.

2011

In the second part of this two-part article we continue on our voyage through a transmission line from DC onwards and upwards through the frequency spectrum, step by step exploring the characteristics from very low to ultra high frequencies.

In this two-part article I'd like to join you on a voyage through a transmission line from DC onwards and upwards through the frequency spectrum. In Part 1 we trace the impedance from infinity at DC to the GHz region where it reaches the steady state value of its characteristic impedance.

Sometimes engineering results in some uncomfortable compromises; this is often the case with PCBs as the mathematical methods used by the modelling tools are based on "ideal" physical properties of materials rather than the actual physical materials in use.

At ElectroTest Expo at Bletchley Park, UK, Martyn Gaudion noticed the extent to which some technologies change, while the overall concepts do not. Prospective customers still ask exactly the same questions as they did 50 years ago: “What’s the bandwidth? Will it work in my application? How accurate?” Followed by the predictable, “How much does it cost?”

2010

In the last edition of "The Pulse," we began a discussion on how a modern field solver can help choose the most cost-effective material for a high-frequency application. Last month we looked briefly at the effects of line length and dielectric losses and this month we focus on copper losses; all three are primary drivers for losses.

The EE creating the "platform spec" and the PCB fabricator responsible for its realisation face an array of materials with a mix of choices: From ease of processing to reliability requirements and signal integrity. For then next two months, "The Pulse" will focus on signal integrity, describing how to use field solvers to select the best materials when trading cost versus SI performance.

Three words, or rather, phrases are in the process of entering the vernacular of the PCB industry, albeit one phrase is already familiar, but taking on a different meaning. All start with S and all relate in one way or another to signal integrity.

In his 1974 philosophical novel "Zen and the art of Motorcycle maintenance” Robert M. Prisig contrasts his regular and ongoing daily approach to motorcycle maintenance with his friend's alternate view of leaving well alone between annual service center based maintenance. What has this got to do with accurate impedance measurement you may ask? Please read on to discover more…

Polar Instruments CEO Martyn Gaudion will be exploring a number of themes. A major SI topic that is set to grow is the emergence of new silicon families designed to push traditional materials into the multi-gigahertz arena. These new chipsets lift transmission speeds up to a point where signal losses rather than reflections become the predominant concern from an SI perspective.